A light emitting device using a light emitting diode (LED) is broadly used for a lighting system such as a backlight of a liquid crystal display apparatus, a signal apparatus, switches, a lamp for vehicle, or a general lighting. In particular, a white light emitting type LED lamp (white LED lamp) made by combining an LED and a phosphor attracts attention, in addition to as a backlight of a liquid crystal display apparatus or a lamp for vehicle, as a substitute of a lighting system using a white lamp, a fluorescent lamp, or the like and its practical application as a white lighting system is being proceeded.
As the white LED lamp, there are known an LED lamp made by combining an LED chip emitting blue light and a yellow phosphor, an LED lamp made by combining an LED chip emitting blue light, a green to yellow phosphor, and a red phosphor, an LED lamp made by combining an LED chip emitting ultraviolet to violet light with an emission wavelength of about 360 to 440 nm and a mixture (BGR or BYR phosphor) of respective blue, green to yellow, and red phosphors, and so on. As the green to yellow phosphor used for the white LED lamp, there is known an europium (Eu) and manganese (Mn)-activated alkaline earth silicate phosphor such as (Sr, Ba, Mg)2SiO4:Eu, Mn.
Though the white LED lamp made by combining the LED chip emitting blue light, the green to yellow phosphor, and the red phosphor is superior to the white LED lamp made by combining the LED chip emitting blue light and the yellow phosphor in a light emission efficiency and a color rendering property, but has a flaw of being inferior in a weather resistance. In particular, under a high-temperature/high-humidity environment, deterioration of a luminance of the white LED lamp becomes notable. The above is considered to be derived from Eu and Mn-activated alkaline earth silicate phosphor used as the green to yellow phosphor. Also in the white LED lamp made by combining the LED chip emitting ultraviolet to violet light and the BGR or BYR phosphor, reduction of a weather resistance becomes a problem when the Eu and Mn-activated alkaline earth silicate phosphor is used as the green to yellow phosphor.
Also inferior in the weather resistance are an europium (Eu) and samarium (Sm)-activated lanthanum oxysulfide phosphor such as La2O2S:Eu,Sm used as a red phosphor, a copper (Cu) and aluminum (Al)-activated zinc sulfide phosphor such as ZnS:Cu,Al used as a green phosphor, and a silver (Ag) and aluminum (Al)-activated zinc sulfide phosphor such as ZnS:Ag, Al used as a blue phosphor. Luminance deterioration of the white LED lamps becomes a problem also in a case that the above phosphors are used. In order to improve a luminance maintenance ratio under a high-temperature/high-humidity environment of a light emitting device such as a white LED lamp, it is required to heighten weather resistance of an alkaline earth silicate phosphor used as a green to yellow phosphor, a lanthanum oxysulfide phosphor used as a red phosphor, and a zinc sulfide phosphor used as a green phosphor or a blue phosphor.